Electron probe microanalyzers and electron microscopes having an attached x-ray spectrometer are used to determine the composition of microscopic or nanoscopic regions of a surface. The detectors determine the energy or wavelengths of x-rays emitted from the sample and infer the composition of material under the electron beam from the energy or wavelength of the x-rays. Detectors that use a crystal to disperse and analyze x-rays of different wavelengths are referred to as wavelength dispersive spectrometers (WDS) and detectors that measure the energy of incoming x-rays are referred to as energy dispersive spectrometers (EDS). While a WDS can provide better resolution and faster counting for a particular wavelength of x-ray, an EDS is better adapted to measuring x-rays of different energies from multiple elements.
Two types of semiconductor energy dispersive x-ray detectors are commonly used in electron microscopy: lithium-drifted silicon detectors “Si(Li)” and silicon drift detectors “SDD”. Si(Li) detectors typically require cooling to liquid nitrogen temperatures and normally have a standardized active detection of area of 10, 30 and 50 mm2. SDDs can operate at a higher temperature and can provide better resolution at high count rates. To avoid ice formation and contamination on the detector, as well as damage from backscattered electrons, a window of a light element such as berrylium is often attached in front of the detector to stop the electrons. A magnetic field can also be used near the detector entrance to divert electrons away from the detector. A collimator is often used in front of the detector to reduce x-rays from sources other than the sample from entering the detector. Some detectors, such as the one described in U.S. Pat. No. 5,569,925 to Quinn et al., include a shutter in front of the detector. When the electron microscope is operated under conditions that would generate high energy x-rays and electrons that could damage the detector, the shutter can be closed to protect the crystal.
Ice formation is also reduced by providing a colder surface near the detector. For example, in the system described in U.S. Pat. No. 5,274,237 to Gallagher et al. for a “Deicing Device for Cryogenically Cooled Radiation Detector,” the heat generated by the detector circuitry maintains the detector a few degrees warmer than the collimator surface so that moisture sublimes from the detector surface onto the collimator surface. The heat generated by the circuitry provides a temperature difference of only about five degrees, which may not be adequate to maintain an ice-free surface on the detector. U.S. Pat. No. 4,931,650 to Lowe et al. for “X-ray Detectors” describes periodically heating the detector above its operating temperature while maintaining a heat sink at operating temperature. Periodically heating the detector above its operating temperature does not stop the build-up of ice during operation and requires periodic interruption of the system operation to remove the ice.
For greatest sensitivity, the detector should cover a large solid angle from the sample to collect as many of the emitted x-rays as possible. To increase the solid angle, the detector can provide a larger active surface area, or be placed closer to the sample. In a transmission electron microscope, the pole pieces and sample holder take up most of the space around the sample and it can be difficult to position X-ray detectors close to the sample to increase the solid angle. U.S. Pat. No. 4,910,399 to Taira et al. teaches a configuration that puts a detector closer to the sample and allows the detector to subtend a larger solid angle. Another configuration is shown in Kotula et al., “Results from four-channel Si-drift detectors on an SEM: Conventional and annular geometries,” Microscopy and Microanalysis, 14 Suppl 2, p. 116-17 (2008). Kotula et al. describe a four-segment detector, with each segment being kidney-shaped and having an active area of about 15 mm2. The detector is positioned above the sample below the pole piece of an SEM, with the four segments distributed in a ring that is coaxial with the electron beam. This configuration is not normally possible in a high-resolution TEM.